The present invention relates to an offset calibration system and method of an automatic power control loop for calibrating the offset generated by the automatic power control loop.
When the pickup head of an optical information recording/reading apparatus writes and reads pits of a compact disk, an automatic power control loop is used to control and adjust the laser power of the laser diode within the pickup head. A pickup head comprises a laser diode, a laser diode driver and a laser power detector. The laser diode provides a laser source to read or write data on the compact disk. The laser power detector detects the laser power of the laser beam to output a corresponding laser power detection signal.
The pickup head uses different laser powers when it is writing or reading. The power in writing is higher than the power in reading. It is important to control the power of the laser emitted by the laser diode. The laser diode is driven by the laser diode driver. Hence, controlling the laser diode driver can further control the laser power.
The automatic power control loop controls the laser diode driver to output an electric current to drive the laser diode for emitting a laser bean with an adequate laser power. The laser power detection signal outputted by the laser power detector and the laser power of the detected laser beam are in inverse proportion. The laser power detection signal will be sampled and held by a sample/hold circuit of the automatic power control loop, and then transmitted to a pre-amplifier for amplifying and generating a pre-output signal. The pre-output signal is subtracted by a predetermined reference signal in an adder. If the result value does not equal to zero, the laser power of the laser beam still needs to be adjusted. A post-amplifier can adjust the pre-output signal and generate a power control signal. The post-amplifier transmits the power control signal to the laser diode driver for emitting an adequate laser power. In an ideal condition, when the laser diode doesn""t emit the laser beam, the pre-output signal is the predetermined reference signal.
In general, the prior art control and calibration method for the laser power emitted by the laser diode can adjust the laser power very well. However, in the prior art, when the laser power detector generates the laser power detection signal with an offset, the automatic power control loop can""t calibrate immediately. Through the amplifier, the offset is amplified and may be beyond the range that the adder can adjust. Therefore, a wrong power control signal is produced resulting in a wrong laser power. Consequently, mistakes occur in writing/reading data of the compact.
It is therefore a primary objective of the present invention to provide an offset calibration system and method of an automatic power control loop for calibrating a laser power emitted from a laser diode so that the laser diode can emit accurate laser power.
The automatic power control loop of the present invention comprises a pre-amplifier circuit and a post-amplifier circuit. The pre-amplifier circuit generates a pre-output signal and then transmits the pre-output signal to the post-amplifier circuit for amplifying. The offset calibration system comprises a comparator, an offset calibration module and a determination circuit. The comparator compares the pre-output signal with a predetermined pre-reference signal to generate a pre-compared signal. The offset calibration module stores a predetermined and adjustable offset calibration value for adding to the pre-output signal. The determination circuit electrically connects to the comparator and the offset calibration module to adjust the offset calibration value stored in the offset calibration module based on the pre-compared signal generated by the comparator.
The determination circuit adjusts the offset calibration value dynamically based on the pre-compared signal, so the offset calibration value can be added to the pre-output signal dynamically to compensate and calibrate the offset of the pre-output signal caused by the pre-amplifier circuit. That will avoid the post-amplifier circuit amplifies the offset of the pre-output signal again.
The present invention calibrates the offset of an automatic power control loop by the offset calibration system. The offset of the pre-output signal can be compensated and calibrated immediately by the offset calibration system of the present invention to avoid that the post-amplifier circuit amplifies the offset of the pre-output signal again.
These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after the following detailed description of the preferred embodiment, which is illustrated in the various figures and drawings.